Make Taskbar focusable when a popup is open.

- Added ArrowPopup#OnPopupClosedCallback to automatically remove Taskbar focus when the popup closes.

Fixes: 209917078
Test: opened popup in taskbar and closed it with switch access, touching anywhere on the screen and using the back gesture. went home to check if focus was removed.
Change-Id: Ie7aafc9cf0f03fadaa44e77818508e9e1d8db610